In the field of printing, it remains desirable for printers to accurately and precisely portray colors. If a printer fails to apply the correct amounts of colorant (e.g., ink, toner, etc.) to a print medium (e.g., paper), then colors may appear improperly shifted in hue, certain colors may have unexpected changes in brightness, and the printed output may be different than expected. For example, photographic images printed by a printer that has not been properly calibrated may appear unrealistic because the colors generated by the printer do not match the colors intended for printing.
In order to address these issues, printers are typically calibrated and then profiled in order to ensure that their printed colors accurately match the colors indicated in print data. Typically, the process of external calibration is performed via a spectrophotometer. The printer places a calibration patch onto the print medium for each of its loaded colorants (e.g., cyan, magenta, yellow, and black). Each calibration patch includes multiple sections of different intensity, and each section includes colorant that is substantially uniformly dispersed onto the medium. The spectrophotometer analyzes the spectral response of the sections of each calibration patch, and a determination is made as to whether each section sufficiently matches an expected color. If the color for a section is not accurate (i.e., is too bright, too dark, not the correct hue, etc.), then halftone patterns used to apply the colorant to that section may be adjusted accordingly. For example, if a section of colorant is expected to be an intense yellow, but is not sufficiently yellow as applied to the page, a different halftoning pattern can be used by the printer to apply the colorant more densely. This disperses more colorant onto the printed medium and makes the section appear more deeply yellow. Once the calibration has been completed, the printer may accurately apply each colorant at the appropriate levels to generate desired colors. Profiling may then be performed in order to determine how to accurately blend different colorants to form new hues.
The calibration and profiling of a printer is not simply a process that can be performed once and then forgotten. As environmental conditions vary, and as the printer itself is exposed to wear and the passage of time, the printer may need to be re-calibrated and profiled on a regular basis.
Presently, many forms of print media include optical brighteners that enhance their perceived white color and brightness. These optical brighteners actually increase the reflectance of the medium with regard to blue and ultraviolet wavelengths of light (e.g., wavelengths roughly between about 390 and 480 nanometers). Because optical brighteners add a blue component to the light that they reflect, optical brighteners can impact the way that colorants applied to the medium are measured by a spectrophotometer. This is particularly true for yellow colorants, because yellow is complementary to blue. This is also particularly true when small amounts of colorant are applied to a section of print medium in order to generate pale or light colors.
For example, in order to create a portion of what appears to be continuous color on a print medium, printers typically apply halftone patterns of pels that are placed onto the print medium in a uniform manner. Each pel within the halftone pattern is separated from its neighbors by blank portions of the print medium that remain uncovered by the colorant. Because the optical brighteners in the blank portions are slightly blue, the overall hue of the halftone pattern of colorant applied to the print medium skews closer to blue. Even when colorant is applied so as to completely cover the print medium, most colorants are translucent and therefore the blue color may continue to shine through the colorant. This makes the printed result appear slightly blue and less like the color intended for printing.
FIG. 1 illustrates the discussion provided above. As illustrated in FIG. 1, a print medium 100 includes a single pel of colorant, indicated as droplet 110, which is partially translucent. Normally, incident light 102 that is applied to medium 100 is reflected by medium 100 as light 104, which may then be measured by a spectrophotometer. The wavelengths of incident light 104 that are reflected naturally depend on the characteristics of medium 100 itself. Medium 100 is also likely to allow a certain amount 106 of incident light 102 to pass through it. Similarly, for droplets of most colorants, incident light 112 is partly reflected by the colorant itself as light 114. Additionally, a certain amount of incident light 112 passes through the colorant of droplet 110, and is reflected by medium 100 as light 116. Still further, a certain amount 118 of incident light 112 passes through both medium 100 and droplet 110. Thus, the light measured by the spectrophotometer includes light 114, which has been reflected by the colorant, as well as light 116, which has been reflected by medium 100. When optical brighteners are used, light 104 from non-colored sections of the page, as well as light 116, may skew the color of a given section of a page towards blue.
As presently practiced, it is undesirable to use a spectrophotometer to calibrate print media that include optical brighteners. This is because for yellow colors, the calibration process may attempt to completely cover the paper with yellow colorant in order to compensate for the blue color of the print medium and make the section look less green (assuming the calibration process does not fail entirely). This is unacceptable because such a calibration process turns pale yellows into intense yellows, resulting in inaccurate representations of color. Thus, an alternative method is presently used for calibration of print media that include optical brighteners. The alternative method involves using a UV-cut filter to substantially eliminate the measured reflectance of a calibration patch for blue wavelengths of light. This allows for the calibration to be completed without error, but unfortunately can result in inaccurate color calibration, because the calibration process does not take into account contributions to hue and intensity caused by reflected blue light.